Is money taken from the company considered lawful (halal) if the number of compensatory days for travel is inflated, given circumstances such as not receiving the agreed-upon salary, discrimination in compensation among employees, and working overtime without pay?

You are not permitted to write that the number of travel days is seven if it is five, because that is a lie and deception, and taking money unjustly. The fact that some foreigners charge a higher wage does not justify lying. If the promised salary is the agreed-upon contract, then they are not allowed to break it, and you have the right to demand the remainder. If they refuse, you may take it even without their knowledge. However, if it was merely a promise of an increase and they did not fulfill it, you are not permitted to take more than what was agreed upon. Regarding working on the weekend, if it is included in the contract for the same salary, then it is not permissible to take an increase. If it is not included, then you have the right to refuse or request compensation.